1. Field of the Invention
This invention relates to DC-to-DC converters that process electrical power from a source, and more particularly, to a variable voltage demagnetization circuit for use with a DC-to-DC forward converter.
2. Description of the Prior Art
In a DC-to-DC forward converter, a power transformer is simultaneously connected to a source and a load. The transformer of the forward converter must completely demagnetize so that the transformer will not become saturated, and hence, inoperable. That is, V.sub.s .times.t.sub.on =V.sub.dem .times.t.sub.off were V.sub.s is the source to the transformer during the time t.sub.on and V.sub.dem is the demagnetizing voltage applied during the time t.sub.off. The products V.sub.s .times.t.sub.on and V.sub.dem .times.t.sub.off represent, respectively, the energy stored in the transformer, and the energy released therefrom.
Conventional demagnetization topologies embodied in circuits provide demagnetization voltages that are either directly proportional to the input voltage, or are fixed. In either case, a demagnetization voltage higher than the theoretically necessary minimum voltage is generated, and necessarily imposes a high rating requirement for the switching transistor. The rating of the switching transistor is considerably higher than would be the case if for example the demagnetization voltage could be controlled to be were inversely proportional to the input voltage.
A low rating for a switching transistor in a forward converter is desirable because, in some instances, transistors are unavailable in the desired range. In certain applications, such as in military specifications, a high margin of reliability is required. In order to meet these requirements, a lower rating transistor must be used.
Another problem characterizing forward converters results from the leakage inductance between the windings of the transformer. The sudden interruption of the current in the primary-to-secondary leakage inductance, which occurs when the switching transistor in the forward converter turns off, creates an overvoltage condition across the switching transistor. This overvoltage can be damaging to the transistor and can be effectively clamped only with a complex circuit. Naturally, this overvoltage is undesirable.